Expanding serial ports

Question. Most of the available boards have 4-6 serial ports available. Let’s say, I have a Kakute F7 - it’s got 7. Ok, 1 is for ESC telemetry, one is for RX, one for GPS, one is for USB and is inaccessible. 3 left. Typically you’d want a mavlink telemetry and if it’s a camera drone gimbal control. One port left - and many options to connect - ADS-B, Companion computer, OSD, flow sensor, rangefinder(s) and such. So, the question is, is there any way to expand number of ports (by using an external device maybe?) or convert those devices to I2C or similar and daisy-chain them somehow? Is it possible to have a secondary FC to help with that maybe?
What do you do if you want a bunch of sensors?

A few options. UAVCAN would be one option. OlliW’s UAVCAN for Hobbyists project is a great resource if you’re interested in taking that path.
There’s also a thread here somewhere from @ppoirier that details an option he has implemented using an arduino or teensy as a sensor hub.
Alternatively, for mavlink enabled peripherals you could use a companion computer with apsync to route multiple mavlink devices.

CANbus for Ardupilot

Interesting, thanks. So, theoretically, using a Bluepill approach from Olliw’s webpage it’s possible to hook up few devices to a single bus…

Yes, but be aware of bandwidth limitations of uavcan.